Houben had heard the doctors, nurses and family speaking in his room for decades.

"I shall never forget the day when they discovered what was truly wrong with me -- it was my second birth," Houben told the German magazine Der Spiegel, communicating via a special keyboard.

Laureys discovered Houben' state in a Belgian hospital three years ago, but the case has only just come to light after Laureys published a study in the journal, BMC Neurology earlier this year. Laureys said in recent study of his, "4 out of 10 patients where we thought they were in a vegetative state in reality were not, [they] were minimally conscious."

"In Rom's case, it was very dramatic, because he was fully conscious… so, yes he now can actually tell us how horrible it was and it's hard for us to even imagine what it must be like.... to be conscious yet, cut off from the outside world," said Laureys.

He argued that new imaging technology will show that many more people like Houben, believed in a vegetative state are actually misdiagnosed.

According to ABC News senior health and medical editor Dr. Richard Besser, misdiagnosis can happen fairly easily in the beginning of treatment.

"When you have a major accident, an injury, you can damage the entire brain," Besser told "Good Morning America." "Over time, parts of it can come back. The damage to the outer brain, that gets better [and] they don't do additional testing."

More often than not it's a family member that realizes the patient is not in a vegetative state just from being with them in the room, Besser said.

But until Laureys' research moves out of the experimental phase and into standard testing at hospitals, experts say there may be some rare situations where doctors still miss the signs that someone is completely paralyzed but fully aware.

"If there's anything we've learned in the past couple of years, is that we have to be very cautious about prognosticating; providing a severe prognosis early on," said Dr. Michael DeGeorgia, director of the Neurocritical Care Center at University Hospitals Case Medical Center in Cleveland.

Other Silenced, Paralyzed Patients Out There

DeGeorgia said determining a person's state of consciousness after a severe brain injury can be a subtle and difficult process, especially shortly after an accident when people are often on heavy pain medication. Now there is an entire sub-specialty of neurology to look at "neurocritical care," but not when Houben was diagnosed.

Without the modern functional imaging technology Laureys used, DeGeorgia said doctors more often relied on a "primitive" test called the Glasgow Coma Scale, a 15-point check list of eye and motor movements in response to questions.

Cindy and Steve Chiappa of Toms River, N.J., found themselves in a similar situation eight years ago, after Steve suffered a stroke on Christmas Eve.

How Doctors Tell If You Are 'Vegetative' or Not

Chiappa is still too paralyzed to speak clearly, but has now progressed to the point where he can eat a variety of soft foods and communicate through a specialized keyboard.

"Other than shining a light in my eyes and asking me to follow the beam they didn't ask me too much," Chiappa wrote in an e-mail to ABCNews.com.

For the first few weeks after he woke up in the hospital, only his family believed that he was actually aware of his surroundings.

"They would ask simple things like, 'If you know who you are, blink' but my response time was so slow, by the time I could blink my response they had turned away," wrote Chiappa.

Convincing Doctors That You're 'There'

"He was scared; the doctors did not want to believe it. It took about a week-and-a-half for me and all our kids badgering the doctors for a diagnosis," said Cindy. "Even three months later, they said, 'oh, he's just going to be a vegetable and that's it.'"

Finally, with a little help from a nurse who goaded the doctors, Chiappa was diagnosed as being "locked-in" rather than a vegetable. A massive stroke that hit the base of his brain had left him unable to move, but left his mind perfectly intact.

Neurologists say paralysis plays only one role in a complicated combination of impaired states that can trap a person in their body.

Difference Between Vegetative and Conscious State

For example, a person who is in a vegetative state and a person who is "locked-in" would both be paralyzed to some degree. Both patients would likely open their eyes and look around.

But, as Dr. James L. Bernat of the American Academy of Neurology explained, these two individuals would be considered to have two different definitions of "consciousness."

"One is called wakefulness; eyes open, eyes moving -- that element is conducted primarily by the brainstem," Bernat said. "The second dimension of consciousness requires self-awareness -- they're aware of what's going on, they can feel, they can think.

"People in the vegetative state have only the wakeful dimension," he said.

A coma would be a state of full paralysis and full unconsciousness similar to the experience of going under anesthesia. Neurologists even define a third state in people with brain injuries as "minimally conscious," which is a state of semi-wakefulness and limited self-awareness.

In a minimally conscious person, "there may be parts of the brain that are able to generate certain types of thoughts similar to what a conscious person would do, but they're still quite devastated and quite injured," said Dr. Paul M. Vespa, director of Neurocritical Care at the David Geffen School of Medicine at the University of California, Los Angeles.

"There are probably a very small number of patients who are in this minimally conscious state. The exciting thing is, is that maybe there's a potential for rehabilitation," said Vespa.

Researchers, like Laureys, will have to do more work to define which brain image results from a functional MRI or PET machine are signs of a response, and which are random noises, before such progress could happen.

"We're not at the point where you can just throw somebody in the scanner and come up with the answer," DeGeorgia said. "[We] don't know how to interpret these [scans], because there have only been a few studies, and you have to know what is normal, and what is within the bell-shaped curve."